Various optical systems have been proposed as an optical system which is assembled in a virtual image display apparatus, such as a head-mounted display (hereinafter, referred to as an HMD) mounted to the head of an observer (see PTLs 1 to 4).
In regards to the virtual image display apparatus, such as an HMD, it is desirable to enlarge a viewing angle of video light and to reduce the size and weight. With the reduction in size and weight, it is possible to reduce the thickness in a visual axis direction of the observer and to bring the center of gravity close to the observer, thereby improving the fit.
If the field of view of the observer is completely covered and thus only video light is viewed, an external scene is not noticed by the observer, causing uneasiness for the observer. Furthermore, the external scene and video are displayed in an overlapping manner, various new usages, such as virtual reality, are produced. For this reason, there is a need for a display which displays video light in an overlapping manner without obstructing the field of view of the outside.
In order to improve the fit for the observer and to improve the appearance of the form, it is desirable to arrange a video display device across the face, instead of being disposed above the eyes.
In order to reduce the size of the optical system and to arrange the video display device in a position distant from the eyes of the observer so as not to obstruct the field of view, it is preferable to image display image light on the optical system once to form an intermediate image, and a relay optical system which enlarges the intermediate image to be displayed.
For example, PTL 1 proposes a relay optical system which uses a parallel planar light guide plate, in which a parabolic mirror is provided at an end surface thereof, and a projection lens, and forms an intermediate image inside the light guide plate. However, in case of the optical system of PTL 1, the projection lens is large in size, obstructing reduction in size and weight.
PTL 2 proposes an optical system which uses a light guide member having a curved emission and reflection surface, and a projection lens which makes video light enter the light guide member. However, the optical system of PTL 2 has no consideration of displaying an external scene to the observer. In order to display the external scene by applying the optical system, it is necessary to attach a compensation member to a reflection surface extending throughout the light guide member and to provide a half mirror on the bonding surface. However, since video light is reflected by the half mirror surface twice, video becomes very dark.
PTL 3 proposes a relay optical system which has a projection lens, a concave mirror, and a light guide plate. In this optical system, a wavelength plate and a polarization half mirror are combined, thereby enhancing reflection efficiency. However, in order to display an external scene by applying the optical system of PTL 3, it is necessary to attach a compensation lens to the outside of the concave mirror, resulting in an increase in thickness as a whole.
PTL 4 proposes a relay optical system which has a short total length and a compact arrangement due to a bent optical path. However, in case of the optical system of PTL 4, a convex portion at the center of the light guide member or a projection lens obstructs the field of view. Furthermore, since video light passes through a half mirror, is then returned and reflected by the half mirror again, and enters the eyes, video to be observed is darkened.